This invention relates to electroconductive composite metal powders, processes for producing the same, an electroconductive paste obtained by using the electroconductive composite metal powders, processes for producing the same, electric circuit devices obtained by using the electroconductive paste, and process for producing the same.
As a process for forming circuit conductors on wiring boards, or on insulating substrates for mounting electronic parts, it has been known to use an electroconductive paste prepared by mixing an electroconductive metal powder (e.g. gold, silver, palladium, copper, aluminum powders) with a binder such as a resin or glass frit and a solvent for forming circuit conductors by coating or printing. The electroconductive paste is applied to through-hole connection, electrode formation, jumper wires, electromagnetic shielding, and the like.
On the other hand, as a process for mounting electronic parts such as resistor elements, chip resistors, chip condensers on circuit conductors, it is known a process comprising coating or printing a solder paste comprising solder particles and a binder, heat treating at a temperature higher than the melting of solder to give an electronic circuit device.
Among various electroconductive metal powders, since gold is remarkably expensive, silver is used as an electroconductive metal powder in a field requiring high electroconductivity and copper is used in other fields. But silver is expensive next to gold and palladium. Further, when a direct current voltage is applied to silver in the presence of water, electrodeposition of silver called "migration" takes place on electrodes and circuit conductors to cause short circuit between electrodes or between wirings. This is a serious problem.
In order to prevent the migration of silver, an electroconductive material containing an alloy of silver and palladium as an electroconductive metal powder is available commercially. But such a material is also very expensive.
On the other hand, copper is cheap and relatively hardly cause the migration. But there is a problem in that when an electroconductive paste is heated, an oxidized film is formed on surfaces of copper particles due to oxygen in the air and the binder to worsen the electroconductivity. Therefore, conductor surfaces are coated with a moisture-resistant paint, or an anticorrosive agent and/or antioxidant are added to the electroconductive material. But sufficient effects are not obtained.
JP-A 56-8892 discloses a process for using silver-plated copper powder in order to improve resistance to oxidation of copper and resistance to migration of silver. But, according to this process, electroconductivity is worsened compared with the case of using silver powders and a part of silver powders is only replaced by copper powders.
JP-A 3-247702 and JP-A 4-268381 propose processes for preparing electroconductive particles by atomizing silver particles on surfaces of copper particles. But these processes are complicated to raise production cost. Further, the resulting powders have a problem in that the resulting powders are almost spherical to give a smaller contact area between powders compared with flat or branched powders and to give a high resistance.
As to the solder pastes, lower heat treating temperature and without use of lead are required seriously recently, but no lead-free solder having sufficiency in melting point and workability has been obtained.